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Abstract
This paper designed and implemented a fiber optic biosensor to detect and measure the refractive index (RI) of different drug samples based on the offset and taper technique and enhance the sensitivity by nanoparticle material coating. This sensor was designed using a coreless fiber (CF), this optical fiber was tapered with different waist diameters, and the optimal waist diameter of 83.06 µm was achieved 291 nm/ refractive index unit (RIU), and the sensor was coated with different concentration of TiO2/PVA the optimal concentration 0.02% wt has a thickness 2.6 µm of TiO2/PVA nanoparticles and it was tested with different drug samples solution with refractive indices ranging from 1 to 1.393 and the highest sensitivity was achieved 361.11 nm/RIU. It was found that the taper and TiO2/PVA nanoparticles improved the sensitivity. This sensor can detect various refractive indices of chemicals and biochemical liquids. Advantages of the proposed sensor include high sensitivity, adaptability, enabling faster real-time measurements, ease of manufacturing and operation, compact size, lightweight design, and low cost.
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1. Introduction
Optical fiber sensors have attracted significant research interest because of their wide range of industrial applications, including chemical and biological detection [1]. Optical fiber sensors have piqued the interest of researchers across multiple sensing disciplines due to their wide-ranging industrial applications, including refractive index (RI) sensors [2], pH sensors [3], strain sensors [4], temperature sensors [5], and humidity sensors [6]. Most optical fiber sensor structures operate on the principle of the Mach-Zehnder interferometer (MZI). MZI, which has been implemented using different fabrication techniques such as; optical fiber tapers [7], thin core fibers (TCF) [8], long-period grating cascade structures [9,10], photonic crystal fiber (PCF) [11], coreless fiber (CF) [12,13], offset technique [14–16], D-shaped optical fibers [17], and U-bent optical fiber sensors [18]. One important aspect of refractive index (RI) optical fiber biosensors is the manipulation of the fiber to enhance the interaction of light with its surroundings. [19]. Tapered optical fibers are a useful technique to adjust the diameter of the cladding zone and allow light from the core to leak into the fiber cladding [7]. Tapered fibers have excellent potential for sensing applications because they enable a greater proportion of evanescent waves to interact with surrounding materials, resulting in high sensitivity [20].
On the other hand, the optical fiber RI biosensor is a structure composed of a single-mode-multimode-single-mode (SMS) and has unique advantages, such as ease of fabrication, low cost, small size, and high sensitivity compared with other fiber structures [19,21,22]. Coreless fiber (CF), a special type of multimode fiber (MMF), is good for fabricating simple, stable, and cost-effective sensors. CF has higher sensitivity than MMF due to the absence of cladding [20]. Thin film coatings combined with tapered optical fibers can produce two different resonance types, surface plasmon resonances (SPRs) [23,24] and lossy mode resonances (LMRs) [24–27]. Surface plasmon resonance (SPR) happens when the real part of the thin film permittivity is negative and higher in magnitude than both its imaginary part and the permittivity of the material surrounding the thin films. On the other hand, the LMR effect can be generated by metal oxide films whose real part of the permittivity is positive and higher in magnitude than both the imaginary part of the permittivity and the permittivity of the surrounding material. The LMR phenomenon has been observed with metal oxides, such as indium tin oxide (ITO) [24,28], titanium oxide [29,30], and indium oxide [24,31]. Resonances in optical experiments are often caused by light interaction with the material structure. Resonance shifts with cladding index change are often measured using optical sensors, such as those based on surface plasmon resonance (SPR) or other evanescent wave-based sensing techniques. The way light travels through the sensor structure is affected by changes in the cladding index, which alters the resonance conditions [32]. Surface plasmon resonance (SPR) is most commonly applied in the visible and near-infrared (NIR) regions of the electromagnetic spectrum.
This study proposed a new structure with an offset and taper technique that was spliced with a small displacement of offset for two similar segments and different taper waist of CF between lead-in and lead-out of single-mode fiber (SMF). The structure was coated with different concentrations of PVA/TiO2 (0.01% wt, 0.02% wt and 0.03% wt to enhance the sensitivity of the sensor. The highest sensitivity was obtained, reaching 361.11 nm/RIU at the best taper waist of 83.06 µm, the best offset of 2.6 µm, and the optimal concentration of TiO2 at 0.02% wt. The best sensitivity was obtained at the smallest size, as the length of the sensor arm was only 20 mm. The small size of the sensing region makes it easier to work with, less susceptible to breakage, less susceptible to breakage, and requires less consumption of coreless optical fiber when fabricating, compared with previous studies. When comparing titanium dioxide to other plasmonic-enhancing materials, it's important to consider various factors such as its optical properties, stability, and ease of preparation in some applications that appear as advantages over other materials. In the case of coating titanium dioxide optical fiber sensors implies that in this application, titanium dioxide might work better than other plasmonic-enhancing compounds. Nevertheless, more investigation and analysis are required to properly understand the benefits and drawbacks of titanium dioxide in contrast to alternative plasmonic-enhancing compounds.
This study showed the enhancement of sensitivity by taper and coating of coreless fiber and achieved a smaller size of the sensor device that is easy to fabricate and carry, and is cost-effective. It can be linked with accurate devices to be used in other applications. The structure was used for biosensor applications to detect the identity of liquid pharmaceutical droplets and syrups, which, when increasing the refractive index, increases the wavelength shifting in a red direction.
2. Material and method
2.1 Fabrication process of sensor
The fabrication process of the proposed single mode-coreless-coreless-single mode (SCCS) structure includes two steps. First, the SCCS was fabricated by a fusion splicer, and then the deposition of PVA/TiO2 onto the taper region of the coreless fiber CF. The proposed structure comprises a two-section of 10 mm length CF (FG125LA from Thorlabs) spliced by offset technique and joined between two SMFs (Corning SMF-28). The core diameter of SMF was 8 µm, while the outer diameter of the SMF and the CF are identical at 125 µm. The fabrication of the SCCS fiber structure was accomplished easily. This is due to the same diameter of the CF and cladding of the SMF. The loss due to the fusion splicing was 0 dB estimated by the splicer. To make the sensor structure more sensitive to detect the changes in the RI, one method is to enlarge the portion of the light that interacts with the surrounding medium. This might be achieved by tuning the thickness of the cladding layer, as a more evanescent field from the cladding penetrates the external medium [14]. Refractive index optical fiber biosensors based on offset, taper, and coating have been proposed and constructed by using a similar coreless optical fiber segment (CF) to enhance the sensitivity of the sensor. The steps of the fabrication sensor are displayed in Fig. 1.
Fig. 1. Steps of fabrication sensor structure (a) offset splicing point between symmetrical CF segments, (b) taper region for CF, and (c) taper region coated with TiO2/PVA.
In this experiment, a fusion splicer was used to reduce the waist diameter of coreless optical fibers. The waist diameter of the CF is controlled by the number of presses on the arc button of the fusion splicing device. As the cladding diameter decreases, the number of cladding modes decreases, resulting in a more primary cladding approach to the surrounding media mode. Furthermore, when the external RI increases, the higher-order cladding modes are no longer guided but radiated for this cladding-reduced structure, which can help to further enhance the sensitivity. The fabricated SCCS structure was tapered using a fusion splicer device. The CF diameter was reduced in the waist from 125 to 72 µm. A microscopic image of CF is represented in Fig. 2.
2.2. Titanium oxide nanoparticles with polyvinyl alcohol coating preparation and deposition
To prepare the titanium oxide nanoparticles with polyvinyl alcohol TiO2/PVA nanostructure film, PVA was used as a host material. PVA as a water-soluble polymer has a good swelling ratio and simple chemical structure of highly OH group bonds. Therefore, PVA swells when it absorbs water molecules from the surroundings [30]. Moreover, a fast equilibrium with atmospheric humidity can be established. PVA refractive index changes with different ambient humidity and it was proved to have good adherence and stability. In the first step, the PVA solution was prepared by dissolving 1 g of PVA powder into 100 ml of deionized (DI) water and mixed with a magnetic stirrer at 90 °C for 1 h until it became a homogenous solution [30], The second step adds different weights of TiO2 nanoparticles powder (0.01, 0.02, 0.03) gm to 10 ml PVA solution with continuous slow stirring using a magnetic stirrer for 3 h the mixture was ultra-sonicated for 15 min. The coating layer was fabricated by immersing the taper region into the TiO2/PVA solution for at least 2 min and left to dry in the air. Figure 3 is a representative field emission scanning electron microscope (FESEM) image of the TiO2/PVA. The TiO2/PVA was uniformly deposited on the surface of the taper region for CF at different zooms.
Fig. 3. FESEM image of the TiO2/PVA deposited on the surface of the taper region for CF at different zooms: (a) 100 µm, (b) 50 µm, (c) 1 µm, and (d) 500 nm.
Energy dispersive spectroscopy (EDS) spectrum to show the materials excite on the taper region of the CF sensor with taper waist 83.064 µm coated with 2.926 µm of TiO2/PVA) was shown in Fig. 4.
Fig. 4. EDS (energy dispersive spectroscopy) spectrum for TiO2 material of coating present on the taper region of the sensor.
(NOTE: The EDS image was taken at the end of the experiment, where there are some traces of the drug solutions that were used in this experiment on the sensor).
2.3 Material samples
The liquid pharmaceuticals were utilized as samples with different refractive indices for drops and syrups. The refractive index for each sample was calculated by the ATAGO refractometer device as shown in Table 1.
Table 1. The refractive index and sample name
2.4 Experimental setup
Figure 5 shows the experimental setup of the proposed sensor to measure the different refractive indices of liquid pharmaceuticals using the taper CF coated with and without the TiO2/PVA film. The set-up consists of the broadband light source (BBS, operating wavelength ranges from 1400 to 1600 nm, Thorlabs SLD1550S-A1) as the light source and an optic spectrum analyzer (OSA: Yokogawa AQ6370C, operation wavelength ranges from 600 nm to 1700 nm) with a resolution of 0.02 nm was used to record the output transmission spectrum. The refractive index of each sample was measured using ATAGO Refractometer where the refractive index ranges from 1 to 1.393.
3. Result and discussion
The proposed enhancement sensitivity of SCCS structures with taper waist (96.2, 83.06, and 72) µm of CF length have been immersed in distilled water (n = 1.333), xylometazoline hydrochloride nasal drops (n = 1.335),diphenhydramine, hydrochloride syrup (n = 1.379), and chlorpheniramine maleate syrup (n = 1.393) with and without the TiO2 coating. The experiment was carried out under room temperature and atmospheric pressure. Figure 6 presents the variation of the transmitted light wavelength through the uncoated SCCS structure for different taper waists for one side and two sides of symmetric segments of CF. The first taper waist 96.2 µm was achieved 244.44 nm/RIU as displayed in Fig. 6(a), and for the second taper waist diameter 83.06 µm was achieved the highest sensitivity 297.77 nm/RIU was displayed in Fig. 6(b). For the third taper waist diameter of 72 µm achieved a sensitivity 224.2 nm/RIU as displayed in Fig. 6(c). Finally, the last structure without coating was tapered with a waist diameter of 83.06 µm from two sides of symmetric segments of CF for the highest sensitivity but the sensitivity decreased from 297.77 to 238.88 with this structure. The wavelength shifting is displayed in Fig. 6(d).
Therefore, this sensor is suitable to use for applications in measuring the refractive index (RI) of liquid drugs. To optimize the sensor performance, sensor characteristics were investigated by depositing the prepared TiO2/PVA film on the taper region of CF. The transmission spectra of the SCCS structure coated by different concentrations (0.01% wt, 0.02% wt, 0.03% wt) of TiO2/PVA at different RI are measured and depicted in Fig. 7(a)–7(c) it is observed that the wavelength of the transmitted spectrum of the coated taper region for CF leaps toward longer wavelengths with an increment of the ambient RI. The wavelength shifting with 0.01% wt concentration of TiO2/PVA for the best taper waist region of CF 83.06 µm achieved sensitivity 211.11 nm/RIU displayed in Fig. 7(a). When the TiO2/PVA coating concentration was adjusted to 0.02% by weight with the optimal taper waist during the experiments, the sensitivity increased from 297.77 nm/RIU to 361.11 nm/RIU. The structure with this concentration achieved the highest sensitivity results in this study. Figure 7(b) depicted the wavelength shifting for 0.02% wt concentration 0.03% TiO2/PVA at tapered waist diameter 83.06 µm where the absence of dips indicates that this concentration of TiO2/PVA is unstable due to the effect the nanoparticles on the evanescent wave and interference modes observed in Fig. 7(c).
Fig. 6. (a) The wavelength shifting for the first taper waist 96.2 µm, (b) the wavelength shifting for taper waist diameter 83.06 µm, (c) the wavelength shifting for taper waist diameter 72 µm, and (d) the wavelength shifting for taper waist diameter 83.06 µm at two sides of symmetric CF segments.
Fig. 7. Wavelength shifting with 0.01% wt concentration of TiO2/PVA for best taper waist region of CF 83.06 µm, (b) wavelength shifting with 0.02% wt concentration of TiO2/PVA for best taper waist region of CF 83.06 µm, and (c) wavelength shifting with 0.03% concentration of TiO2/PVA for best taper waist region of CF 83.06 µm.
With the increment of the refractive index of liquid pharmaceuticals, a remarkable shift of the transmission spectral responses of dip towards the longer wavelengths was noticed. The wavelengths shift to a long-wave direction might be attributed to the increase of the RI of the surrounding medium which reduces the phase difference between the core and cladding modes. The dip was taken as a reference to analyze the spectral response of the proposed RI biosensor. This experiment was divided into two parts, the first was about studying the effect of the tapering on one side, and the second was about the effect of the tapering on two sides of symmetric CF. From tapering on one side where it showed a clear effect on improving the sensitivity performance of the sensor to allow more evanescent waves to interact with the surrounding materials. The summary of this study is displayed in the Table 2.
Table 2 displays the sensitivity for one-side and two-side taper diameters for symmetric CF segments.
Table 2. The sensitivity for one-side and two-side taper diameters for symmetric CF segments
Table 3 displays the sensitivity for different concentrations of TiO2/PVA for the best taper waist region of CF 83.06 µm.
Table 3. The sensitivity for different concentrations of TiO2/PVA.
The performance and dependability of optical fiber sensors are greatly improved by the offset approach, which has several benefits. Numerous industries, such as industrial operations, healthcare, and environmental monitoring, employ optical fiber sensors extensively. These sensors can attain higher levels of sensitivity, accuracy, and stability by applying the offset approach, which results in data that are more accurate and consistent. Table 4 displays a comparison of current sensitivities with previous studies.
Table 4. Comparison of current sensitivities with previous studies.
4. Conclusion
In this contribution, a highly sensitive refractive index (RI) biosensor based on taper SCCS fiber structure in combination with TiO2/PVA thin film acting effectively as a cladding layer has been successfully developed. An experimental RI sensing comparison based on a tapered SCCS fiber structure coated with TiO2/PVA with an identical optical fiber structure but without tapering and coating was carried out. The achieved results revealed that the coated CF at 83.06 µm diameter coated with TiO2/PVA thin film showed much higher sensitivity. The tapered and coated structure with TiO2/PVA achieved higher sensitivity than an identical structure without tapering and coating. It has been proved that the diameter of the CF segment is an important parameter to increase the sensitivity of the sensor device also thin film with high RI plays a role in the sensitivity improvement of SCCS coated devices. The RI biosensor synthesizes the merits of single-mode multimode single-mode (SMS) fiber structure and TiO2/PVA as a smart sensitive material with high flexibility in design and fast response which is important in terms of accuracy in gas and refractive index sensing application. This sensor is simple, has good detection specificity and excellent reliability, it can be used in bio/chemical applications.
Acknowledgments
This work was supported by the Ministry of Higher Education and Scientific Research (MOHESR), University of Baghdad (UoB). This work was approved by the institutional review board of the Institute of Laser for Postgraduate Studies, University of Baghdad.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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